1. Field of the Invention
The present invention relates to an image transferring device for an image forming apparatus such as a copier, printer, facsimile machine or similar photographic image forming apparatus in which an image is formed on a photoconductive element. More particularly, the invention is concerned with an image transferring device for transferring a toner image from the photoconductive element to a sheet of paper which is carried on a transfer belt. The present invention further relates to a method and apparatus for electrically cleaning the transfer belt.
2. Description of the Related Art
It is a common practice for an image forming apparatus of the kind described above to use either a corona discharge type image transferring device or a contact type image transferring device. The corona discharge type device transfers a toner image formed on a photoconductive element to a sheet of paper by effecting corona discharge at the rear of the sheet. The contact type device transfers a toner image from a photoconductive element to a sheet carried on a transfer belt to which an electric field opposite in polarity to the toner image is applied.
The contact type image transferring device usually includes an arrangement for applying a transfer bias to the transfer belt. For example, an electrode is connected to a power source and held in contact with the rear of the belt at an image transfer position. Such an arrangement is advantageous over one using corona discharge, since it does not produce harmful ozone and can operate with a low voltage while reducing the size and cost of the device.
Japanese Laid-Open Publication NO. 5-333717 discloses an image transfer device using a contact electrode as shown in FIG. 12. Referring to FIG. 12, a transfer belt 8 is wound around a driven roller 3 and a drive roller 9, the rollers both being formed of conductive materials. A photoconductive drum 1 is disposed above the transfer belt 8. A conductive bias roller 5 and contact plate 7 are held in contact with the inner surface of transfer belt 8. The bias roller 5 is connected to a power source 18 and is also used as a contact electrode. The transfer belt 8 is made of a dielectric material and has a double layer structure, i.e., a surface or outside layer and an inner layer. The surface layer has an electric resistance of 1.times.10.sup.9 .OMEGA. to 1.times.10.sup.12 .OMEGA. and the inner layer has an electric resistance of 1.times.10.sup.7 .OMEGA. to 1.times.10.sup.9 .OMEGA.. A lever 4 which is driven by a DC solenoid 6 is located under the transfer belt 8. A cleaning blade 11 rubs a surface of the transfer belt 8 and removes residual toner on the transfer belt 8. A toner container 13 and a coil 12 which transports toner to a toner collection container (not shown) are located under the blade 11.
A sheet of paper P is transported to a nip position B between the photoconductive drum 2 and the transfer belt 8 by a pair of a resister rollers 1. At this time the DC solenoid 6 moves the lever 4 which moves the transfer belt 8 toward the photoconductive drum 2 so that the transfer belt 8 is held in contact with the photoconductive drum 2. A transfer bias is applied to the transfer belt via a bias roller 5 so that a toner image is transferred to the sheet of paper P at the nip position B. An electric charge is added to the transfer belt 8 and the sheet of paper P is discharged via a contact plate 7 through the transfer belt 8.
In this case, assuming that an output current from a power source 18 is I1, and a feedback current from the contact plate 7 to a transfer control board 20 is I2. The current I1 is controlled by the transfer control board 20 to satisfy an equation: EQU I1-I2=IOUT
where IOUT is constant.
After the toner image is transferred to the sheet of paper P, the electric charge of the sheet P is discharged gradually by the contact plate 7 via the transfer belt 8 to a ground. Then the sheet of paper P is separated from the transfer belt 8 at the position of the drive roller 9. After the sheet of paper P is separated from the transfer belt 8, the lever 4 is released to separate the transfer belt 8 from the photoconductive drum 2.
After the sheet of paper P is separated from the transfer belt 8, the surface of the transfer belt 8 is cleaned by a cleaning blade 11. The cleaning blade 11 rubs the surface of the transfer belt 8 to scrape off the toner transferred from the background of the photoconductive drum 2 to the transfer belt 8, the toner scattered around the transfer belt 8 without being transferred, and paper dust generated from the sheet of paper P. The toner and paper dust removed from the transfer belt 8 by the blade 11 are collected in a waste toner container (not shown). For this reason, it is required that a coefficient of friction .mu. between the surface of the transfer belt 8 and the cleaning blade 11 be small (0.5 or less) and that there are no cracks on the surface of the transfer belt 8. If the coefficient .mu. is large, it will cause some inconvenience such as an increase in the driving load torque of the transfer belt 8 or a bending of the cleaning blade 11.
After a period of time in the above mentioned transfer device, the frictional coefficient .mu. of the surface of the transfer belt 8 increases, and cracks form on the surface of the transfer belt 8 due to friction between the surface of the transfer belt 8 and the cleaning blade 11. Then, the toner and paper dust cannot be removed from the surface of the transfer belt 8 by the cleaning blade 11. As a result, the reverse side of the sheet P will become dirty and the sheet P cannot always be properly separated from the photo-conductive drum 2.
Japanese Patent Laid-Open Publication No. 3-125372 discloses a bias cleaning device which cleans a residual toner from a transfer belt. This cleaning device can work only when the transfer belt is away from the photoconductive drum in order to prevent an electrical charge from the cleaning bias roller from having a bad effect on the transfer of a toner image to a sheet of paper. Since this device acts only when the transfer belt is away from the photoconductive drum, toner remains on the transfer belt when the sheets are fed successively.